Dark Matter May Not Exist: This Physicist Supports New Theory Of Gravity

By Indranil Banik, Postdoctoral Researcher from Astrophysics, University of St Andrews
10 July 2022

Dark matter is proposed to explain why stars at the far edges of galaxies can move faster than Newton predicted. An alternative theory of gravity may be a better explanation.

Using Newton’s laws of physics, we can model the motion of the planets in the Solar System quite accurately. However, in the early 1970s, scientists discovered that this did not work for disk galaxies – the stars at their outer edges, away from the gravitational force of all matter at their centres – moving much faster than Newton’s theory predicted.

As a result, physicists proposed that an invisible substance called “dark matter” exerted an extra gravitational pull, causing the stars to accelerate – a theory that became widely accepted. However, in a recent review, my colleagues and I suggested that observations at multiple scales are much better explained in an alternative theory of gravity called Milgromian or Mond dynamics – which does not require invisible matter. It was first proposed by Israeli physicist Mordehai Milgrom in 1982.

Mond’s main postulate is that when gravity becomes very weak, as happens near the edges of galaxies, it begins to behave differently from Newtonian physics. In this way, it is possible to explain why the stars, planets, and gas in the periphery of more than 150 galaxies are rotating faster than expected based on their apparent mass alone. However, Mond not only explain such a rotation curve, in most cases, that predict they.

Philosophers of science argue that this predictive power makes Mond superior to the standard cosmological model, which states that there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have very uncertain amounts of dark matter that depend on the details of how galaxies form – which we don’t always know. This makes it impossible to predict how fast the galaxy should rotate. But such predictions are routinely made with Mond, and have so far been confirmed.

Imagine that we know the distribution of visible mass in a galaxy but don’t yet know its rotational speed. In the standard cosmological model, it is only possible to say with certainty that the rotational speed will be between 100km/s and 300km/s at the periphery. Mond made a more definite prediction that the rotational speed should be in the range of 180-190km/s.

If later observations reveal a rotational speed of 188km/s, then this is consistent with both theories – but clearly, Mond is preferred. This is a modern version of Occam’s razor – that the simplest solution is preferred over more complex solutions, in which case we must describe observations with as few “free parameters” as possible. Independent parameters are constants – certain numbers that we have to plug into the equation for it to work. But they are not given by the theory itself – there is no reason they should have any particular value – so we have to measure them observationally. Examples are the gravitational constant, G, in Newton’s theory of gravity or the amount of dark matter in galaxies in standard cosmological models.

We introduce a concept known as “theoretical flexibility” to capture the basic idea of ​​Occam’s razor that a theory with more independent parameters is consistent with a wider range of data – making it more complex. In our review, we used this concept when testing standard and Mond cosmological models against various astronomical observations, such as galaxy rotation and motion within galaxy clusters.

Each time, we assign a theoretical flexibility score between –2 and +2. A score of -2 indicates that the model makes clear and precise predictions without peeking at the data. On the other hand, +2 implies “anything” – the theorist would be able to fit almost any reasonable observation result (because there are so many independent parameters). We also rate how well each model fits the observations, with +2 indicating excellent agreement and -2 reserved for observations that clearly show the theory is wrong. We then subtract the theoretical flexibility score from that for agreement with the observations, because fitting data well is good – but being able to fit anything is bad.

A good theory will make clear predictions which are later confirmed, ideally getting a combined score of +4 on many different tests (+2 -(-2) = +4). A bad theory will score between 0 and -4 (-2 -(+2)= -4). Correct prediction will fail in this case – it is impossible to work with wrong physics.

We found the mean score for the standard cosmological model of -0.25 across 32 tests, while Mond achieved a mean of +1.69 across 29 tests. Scores for each theory in the many different tests are shown in figures 1 and 2 below for the standard and Mond cosmological models, respectively.

Image 1. Comparison of standard cosmological models with observations based on how well the data fit the theory (increasing from bottom to top) and how much flexibility it has (increasing from left to right). Hollow circles were not counted in our assessment, as these data were used to set the independent parameters. Reproduced from table 3 of our review. Credit: Arxiv

Figure 2. Similar to Figure 1, but for Mond the hypothetical particle that only interacts through gravity is called a sterile neutrino. Note the lack of obvious forgeries. Reproduced from Table 4 of our review. Credit: Arxiv

It is immediately apparent that no major issues have been identified for Mond, which at least fits all data (note that the bottom two rows indicating forgery are blank in figure 2).

Problems with dark matter

One of the most striking failures of standard cosmological models has to do with the “stem galaxies” – bright, rod-shaped regions made of stars – that spiral galaxies often have in their central regions (see main image). The bar rotates from time to time. If a galaxy is embedded in a large circle of dark matter, its rods will slow down. However, most, if not all, of the observed galactic trunks are fast-paced. It falsifies the standard cosmological model with very high confidence.

Another problem is that the original models that suggested galaxies have dark matter halos made a huge mistake – they assumed that dark matter particles exerted gravity on the surrounding matter, but were not affected by the gravitational pull of normal matter. This simplifies calculations, but does not reflect reality. When this was taken into account in subsequent simulations, it became clear that the halos of dark matter surrounding galaxies could not reliably explain their properties.

There have been many other failures of the standard cosmological model that we investigated in our review, with Mond often being able to explain observations naturally. The reason why the standard cosmological model is so popular could be due to computational errors or limited knowledge of its failures, some of which were discovered recently. It could also be due to people’s reluctance to change the theory of gravity that has been so successful in many other areas of physics.

Mond’s great advantage over the standard cosmological model in our study led us to conclude that Mond is highly favored by the available observations. While we don’t claim that Mond is perfect, we still think of it as a true big picture – galaxies really lack dark matter.

Written by Indranil Banik, Postdoctoral Researcher from Astrophysics, University of St Andrews.

This article was first published in The Conversation.

Reference: “From the Galactic Trunk to the Hubble Tension: Weighing Astrophysics Evidence for Milgromian Gravity
by Indranil Banik and Hongsheng Zhao, 27 June 2022, Symmetry.
DOI: 10.3390/sym14071331

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